The aim of Total Hip Arthroplasty (THA) is the reduction of pain and the restoration of function to a diseased hip joint via the substitution of engineered materials for the diseased tissue. Successful outcomes depend largely on the proper sizing, placement and orientation of the implant. Incorrect biomechanics (e.g. joint reaction forces, soft tissue balancing) can slow or prevent healing, cause gait abnormalities and lead directly to early implant failure.
There are three portions to the femoral side of a total hip prosthesis: the neck, body, and stem. In a one piece hip design, all three of these portions are a part of a solid construct. As a solid construct, the orientation of each portion with respect to one another is fixed and unchangeable. A surgeon implants one of these solid construct devices by attaching a handle or holder to the prosthesis and then driving it into a prepared opening in the bone. The orientation of the neck and stem is controlled by, and limited to, the orientation of the prepared opening in the bone.
With the invention of two-piece designs, specifically U.S. Pat. No. 4,846,839, issued to Noiles, incorporated herein by reference, and subsequently marketed and sold by DePuy, A Johnson and Johnson Company, under the “S-ROM Hip System” tradename, a surgeon is given the opportunity to independently orient the position of the implant body apart from the rotation or anteversion of the neck. In these designs, the stem and neck of the implant are a one-piece component and the body of the implant is a separate component. These devices also incorporate flutes or spines on the stems of the implant so as to provide additional resistance to rotation. These spines engage the bone during the initial few inches of insertion into the femoral canal, forcing the final location of the implant neck to be totally dependent on the initial insertion orientation of the distal stem. If the stem is fully inserted and the neck is incorrectly oriented, the entire implant must be removed, indexed, and reinserted. This iterative approach to neck placement is time consuming, damages the bone/implant interface, and results in the loss of implant stability. This problem is compounded when a bowed stem is used in place of a straight stem. The bow in the stem will follow the natural bend in femoral canal, which in turn dictates the final position of the femoral neck. Using two-piece modular implants, where the first piece is a neck with curved stem component and the second piece is the body component, a surgeon is given no choice or option for the femoral neck orientation with respect to the orientation of the bowed or curved stem.
With a three-piece modular implant, there is provided a stem component, a neck component, and a body component which are independent of one another and are independently adjustable with respect to one another. With insertion, the stem component and body component each seek a best fitting position within the femoral canal. Following insertion, the neck component can be oriented by a surgeon in order to establish optimal joint and motion mechanics.
Prior to insertion of an actual implant, the use of a mock implant or “trial” is desirable as a means of evaluating correct size and positioning. A surgeon uses the measurements taken from the trial implant so as to select the final implant components. A trial stem is generally smaller than an implant stem and is configured without spines or flutes so as to aid in the insertion and removal of the trial without damaging the bone/implant interface. When inserted into the prepared cavity in the bone, the stem component will follow the path of least resistance so long as its motion is not restricted otherwise. For this reason, it is advantageous to allow the implant stem rotational freedom with respect to the body when inserting the final implant. Although it is desirable for the stem component to be rotationally free during insertion, it is important for the stem component to be properly aligned with the body and neck components so as to ensure proper final assembly. Without this alignment, the stem may be off axis, making it difficult or impossible to assemble the neck and body segments along an axis.
Other methods for implanting two-piece or three-piece modular devices where the stem component is separate from the body and/or neck component(s) include inserting the stem component until it is proud of its intended final position and then using the body component to seat the stem by impaction. For example, a two-piece system having separate stem and combination body/neck components is manufactured and sold by the Biomet Corporation, under the “Impact Hip System” tradename. For further example, a three-piece system having separate stem, body, and neck components is manufactured and sold by the Exactech Corporation, under the “Accumatch M-Series Hip System” tradename. In both examples, impaction of the body onto the stem also serves to secure the stem and body assembly. One disadvantage of this method is that stem/body orientation must be determined visually prior to full seating and may shift during implantation. Another disadvantage is that the assembly force is equivalent to the resistance of the stem sliding down the canal. If the stem moves in the canal during final assembly, the stem may be driven too deeply into the canal causing surgical delays, or the stem and body will not be tightly assembled, leading to fretting, corrosion, and early failure of the construct.